The basic image-forming process of color photography comprises the exposure of a silver halide photographic recording material to light, and the manifestation of a usable image by the wet, chemical processing of the material. The fundamental steps of this processing entail, first, treatment of the recording material with a color developer wherein some or all of the silver halide is reduced to metallic silver while an organic dye is formed from the oxidized developer, and, second, the removal of residual silver halide and metallic silver by the post-development steps of bleaching and fixing. Herein, metallic silver is first oxidized by an oxidant in the bleach, and the silver ion derived from the bleach as well as the residual silver halide are converted to a soluble silver complex by the action of the fixing agent, and it is dissolved away.
It is highly desirable to process a photographic recording material as rapidly as feasible, and an accelerated process----a shortened process compared to ones known in the art----is highly desired. In particular, shortening the silver removal step, which consumes almost half of the total process time, appears to be an attractive manner in which to shorten the overall processing time.
Juxtaposed to the desire for an accelerated process is the desire for, and the need for, photographic recording materials and process solutions that require lower chemical usage and that generate less polluting chemical waste. Used or spent fixer baths are desilvered by electrochemical means to recover the semi-precious metal, but these solutions may still contain ingredients that limit their direct discharge into public waste water streams. Further treatment of these effluent, desilvered fixers may be required for them to conform to standards for environmentally acceptable disposal. In some photographic processes, higher processing speed can be traded for indirect advantage, in the form of lower processing chemistry usage and lower processing waste per unit of photographic material processed. Thus, these desires or goals are interrelated.
A wide variety of fixing agents and silver solvents are known. Such materials form relatively stable and soluble reaction products with silver ion or with silver halides. Such agents include, for example, alkali metal and ammonium thiosulfates, thiocyanate salts, sulfites, cyanides, ammonia and other amines, imides as described in U.S. Pat. No. 2,857,274, thiols as described in U.S. Pat. Nos. 3,772,020 and 3,959,362, thioureas, thioacids, and thioethers as described in German Offen. 2,037,684 and U.S. Pat. Nos. 2,748,000 and 3,033,765, phosphines as described in U.S. Pat. No. 3,954,473, and concentrated halide solutions as described in U.S. Pat. No. 2,353,661.
Examples of fixer bath formulations comprising such fixing agents are many, and may be found in Encyclopedia of Practical Photography, Vol. 6, Eastman Kodak Co., ed., Amphoto, Garden City N.Y., pp. 1086-1091; Photographic Processing Chemistry, Focal Press, London, 1966; Processing Chemical and Formulas, Publication J-1, Eastman Kodak Co., 1973; Photo-Lab Index, Lifetime Edition, Morgan and Morgan, Inc., Dobbs Ferry, N.Y., 1987; and Imaging Handbook of Photography and Reprography Materials, Processes, and Systems, Van Nostrand Reinhold Co., 7th Ed., 1977. Fixer bath formulations may also be found in the references cited in Research Disclosure, Item No. 308119, December 1989, pp. 1010.
Thiosulfate salts are generally preferred as fixing agents because they are inexpensive, highly water soluble, non-toxic, non-odorous, and stable over a wide pH range in the fixer bath. Furthermore, thiosulfate salts form very stable, water soluble reaction products with both silver ion and with silver halides. In addition, these soluble reaction products remain stable under the more dilute conditions of subsequent washing or stabilizing steps of the processing operation, thereby preventing reprecipitation of the silver salts in the film. Lastly, these thiosulfate salts are relatively inert toward the organic photographic image dyes and the gelatin that comprise color photographic recording materials.
It is well appreciated in the art that the aforementioned benefits of thiosulfate fixers are even enhanced in the presence of ammonium ion. Fixer baths containing ammonium thiosulfate are more active and solubilize silver halide in a photographic recording material more rapidly than thiosulfate salts of other cations, such as sodium thiosulfate or potassium thiosulfate. It is appreciated that adding ammonium salts to sodium thiosulfate fixer baths increases the rate of fixing. Thus, owing to its rapid fixing action, ammonium thiosulfate is widely used as a photographic fixing agent.
Ammonium ion is, however, an environmentally detrimental, polluting chemical. The concentration of ammonium in waste water effluent streams is regulated in certain locales, and the allowable limits will likely decrease in the future. Therefore it is desirable to produce a fixing bath that has lower concentrations of ammonium, or that has no ammonium whatsoever, in order to reduce or completely eliminate its contribution to photographic effluent pollution.
If the fixability of the color photographic recording material could be improved, it would be possible to reduce the fixer bath ammonium content and make its effluent less polluting, without increasing the fixing time.
It is appreciated in the art that lowering the silver halide coverage of a photographic recording material will increase its rate of fixing. It is also appreciated in the art that lowering the iodide coverage of a silver bromoiodide photographic recording material will increase its rate of fixing. However, both of the aforementioned methods generally produce inferior quality in the resultant photographic recording material. The unrelenting demands for improved granularity in modern films result in use of high coverages of silver halide emulsions, such that any meaningful silver coverage reduction will in turn increase the film's graininess. Likewise, in the design of practical silver bromoiodide emulsions for color negative systems, it is already desirable to minimize iodide content to maximize emulsion developability and contrast. Further reduction of the iodide content of such silver bromoiodide emulsions typically compromises emulsion quantum efficiency unacceptably, resulting in inferior sensitivity for a given granularity performance.
A very useful way to reduce emulsion granularity for a given speed is by the careful adjustment of emulsion morphology. Employment of high aspect ratio tabular silver halide emulsions, as described in U.S. Pat. Nos. 4,439,520, 4,672,027, and 4,693,954, has succeeded in providing a large variety of advantages to color negative photographic recording materials. Such advantages include improved speed-granularity relationships, increased photographic sensitivity, higher contrast for a given degree of grain size dispersity, higher separations of blue and minus blue speeds, less image variance as a function of processing time and/or temperature variances, the capability of optimizing light transmittance or reflectance as a function of grain thickness, and reduced susceptibility to background radiation or airport x-ray radiation damage in very high speed emulsions.
These interdependent problems of providing a color negative photographic recording material with low granularity, as well as other desirable features of photographic performance, and lower chemical waste per unit of film processed (or alternatively, an accelerated fixing process) would be ameliorated if a photographic material of improved fixability could be utilized, particularly with a fixer bath of improved environmental acceptability.
One approach to improve photographic recording material fixability is to employ fixing accelerating agents either in the fixer bath or in the recording material itself. Compounds which promote or increase the rate of fixing are known, and can be added to the fixing bath. Examples of such fixing promoters are described in Chapter 15 of "The Theory of the Photographic Process", 4th Edition, T. H. James, ed., Macmillan, N.Y., 1977. Such substances include ammonium salts, such as ammonium chloride, ethylenediamine, guanidine, other amines and their salts such as pyridinium and piperidinium salts. Thiourea is also mentioned as a fixing promoter. Many of these compounds are useful only at high concentrations, or they have toxic properties (such as the ammonium ion and amine salts) which make them inappropriate for improved process ecology.
U.S. Patent 4,812,391 seeks faster fixing speed by the utilization of photographic recording materials containing a polymer capable of providing a cation site on the same side of an emulsion layer in a fixing bath. However, there is no indication that incorporated coupler color recording materials obtain satisfactory sensitivity, contrast, and stability in the presence of these polymers, or that adequate fixability is conferred by the presence of these polymers in this instance.
U.S. Pat. No. 4,695,529 describes an image forming process comprising a color photographic material containing tabular silver halide grain emulsions and subjecting the photographic material to color development processing, and then processing said material in a bath having bleaching ability and successively in a bath having bleach-fixing ability. Reduction of ammonium ion content or usage to obtain adequate fixing and lessen the polluting qualities of spent fixer baths is not indicated.
There remains a need for an image forming process comprising a color negative photographic recording material of improved fixability without sacrificing photographic performance and fixer baths that reduce the ammonium ion content of their effluent.